1. Field of the Invention
The present invention relates to digital systems, and more specifically to a device for demodulating a binary phase-shift keyed signal.
2. Description of Related Art
The phase modulation of a signal using encoded pulses is known as phase-shift keying ("PSK") modulation, and when the carrier is a binary signal it is termed binary phase-shift keying ("BPSK") modulation. In BPSK-type modulation. the carrier is a high frequency binary signal and the modulating signal is a low frequency binary signal that has a series of transitions between a high level and a low level for representing an encoded sequence of 0's and 1's. The resultant BPSK modulated signal is a binary signal in which each change in phase corresponds to a downward transition (i.e., from high level to low level) or an upward transition (i.e., from low level to high level) of the modulating signal.
FIG. 1 shows an example of a modulating binary signal S.sub.m (t) with a non-return to zero ("NRZ") type of encoding. As shown, each 1 bit is encoded as a high level for a predetermined period t.sub.m and each 0 bit is encoded as a low level for the same period t.sub.m. The high level and the low level are at +V and -V, respectively. The BPSK modulated signal S.sub.PSK (t) that results when using a carrier at a frequency of f.sub.p is shown in FIG. 2. In a practical example, the modulating signal is at a frequency f.sub.m of 105.9 KHz and the carrier signal is at a frequency f.sub.p of 847 KHz (i.e., f.sub.p is a multiple of f.sub.m to allow coherent modulation). Typically, the first phase change in the modulated signal corresponds to a first downward transition equivalent to an initial bit.
To demodulate such a binary phase-shift keyed signal, the changes in phase in a received BPSK modulated signal must be detected and used to regenerate a demodulated signal that corresponds to the original modulating signal. However, conventional analog-type BPSK demodulation devices use mixers that are extremely bulky and subject to drifts. Additionally, such analog devices are difficult to implement in practice. While it is possible to use an exclusive-or-type digital device and delay circuits, such devices are highly dependent on the modulation frequency (i.e., they must be adapted for each different modulation frequency).